A typical encoder from the prior art is an encoder having a magnetic disk with a polar pitch p, attached to the rotation axis of an electric motor and a circuit board supporting one or two Hall Effect sensors in the vicinity of the magnetic disk, as represented on FIG. 1 or FIG. 2. To enhance the accuracy or resolution of such encoder, the distance between the magnetic disk and the Hall Effect sensor has to be as small as possible. However, due to production variations, and tolerances of the components, a minimal gap is required between the sensor and the magnetic disk. This minimal gap has a negative impact onto the accuracy and is a limit to the reduction of the size of the encoder and of the magnet, as this magnet shall produce a minimal magnetic field greater than the Hall Effect sensor limit of detection. In other words, this minimal gap is a limit to a reduction of the polar pitch, which could lead to a reduction of the packaging of the encoder and/or an increase of the resolution of the encoder.
A reduction of the polar pitch (either by reducing the size of the magnetic dipole or by increasing the number of pole alternances) leads to a reduction of the magnetic field and, as the size of the magnetic sensor remains the same, a negative effect on the sensed signal is obtained because it is not possible to position the magnetic sensor closer to the magnetic dipole of the encoders of FIGS. 1 and/or 2. FIG. 5 illustrates that the magnetic fields strongly decreases with the distance from the magnetic dipole.
Another point to consider is an angular distance between two magnetic sensors as illustrated in FIG. 1 or 2, if the moveable magnetic dipole is a disk. A reduction of this angular distance is desirable, to decrease a phase shift error, therefore, the magnetic sensor of FIG. 2 could be more interesting for this aspect, but there is still the minimum gap between the sensor and the magnetic dipole which is a limit to the encoder downsizing or resolution increase, as being critical for the detection of the magnetic field.